1. Field of the Invention
The present invention relates to Frequency-to-Voltage Convertors (FVC) and, in particular, to a Frequency-to-Voltage Convertor based upon a precision, positive-edge-triggered, amplitude-controlled single-shot having a linear frequency-to-voltage transfer characteristic and that is designed for use in video monitor sync-processing and deflection systems.
2. Discussion of the Prior Art
FIG. 1 shows the general architecture of a phase locked loop (PLL) 10 utilizable in a video monitor sync-processing and deflection system for generating a timing signal OUT that is locked to the frequency of an incoming signal IN. PLL 10 includes a Frequency-to-Voltage Convertor (FVC) 12 that generates an output voltage V.sub.FVC that corresponds to the frequency of the input signal IN. The output voltage V.sub.FVC is provided to a summer 14 the output V.sub.SUM of which controls the output frequency of a voltage controlled oscillator (VCO) 16.
FIG. 1 shows the input signal IN also provided to a phase detector 18 which generates a phase error output V.sub.E that depends upon the phase difference between the input signal IN and the output signal OUT of the VCO 16. If the phase detector 18 is enabled, then its output V.sub.E, filtered by filter 20, is also provided to summer 14 to provide a desired positive or negative correction factor to the output V.sub.FVC of the FVC 12 depending, respectively, upon whether the input signal IN lags or leads the output signal OUT. If the phase detector 18 is disabled then the VCO 16 is driven directly by the output V.sub.FVC of the FVC 12.
One class of FVC utilizable in the FIG. 1 circuit 10 relies on a single-shot that, for each cycle of the input signal IN, generates a pulse of constant duration. Thus, at low input frequencies, the pulses of the FVC output signal V.sub.FVC are sparse; conversely, when the frequency of the input signal is high, the FVC produces a crowded pulsed output signal V.sub.FVC. If the output of this single-shot class of FVC is filtered such that the lowpass filter -3dB frequency is much lower than the lowest frequency of interest in the operation of the FVC, then the resultant DC signal provided to the VCO can be shown to be proportional to the input signal frequency.
However, a single-shot class FVC requires a timing circuit that provides very consistent pulse widths. This consistency is very difficult to achieve because of the matching, temperature compensation and base drive requirements of the bipolar devices typically utilized in such circuits.